The invention relates to a device for sampling a flowing gas.
Many medical and industrial activities require samples of flowing gases for analysis and control of the activity. When the flow of such gases is very small, however, it has been difficult to provide a device which samples the gases accurately and without undesirably interfering with the limited flow of the gas.
In medical activities such as the administration of ventilation or anesthesia gases to patients with an endotracheal tube, for example, the gas exhaled by the patient can be sampled and analyzed for diagnosis or control of the apparatus. With adult patients, the endotracheal tube is large enough to accomodate a coaxial sampling tube, but with infants or neonates, the endotracheal tube is too small for such an arrangement. In addition, the flow of gas from the neonate is so small that the gas sampling apparatus can easily interfere with the flow of gas.
Typically, the necessarily small endotracheal tube for use on a neonate is connected to larger tubing more practical for administering the respiratory or anesthesia gases in a valve arrangement which also vents the gases exhaled by the neonate. Because the flow of exhaled gas is small and the valve arrangement is of relatively substantial volume to be of a practical size, the exhaled gases mix with gases supplied for the previous inhalation cycle in the valve arrangement. Sampling the exhaled gas after it leaves the endotracheal tube thus provides an inaccurate sample. It has therefore been the practice to try to position a flexible gas-sampling tube as close to the connection to the endotracheal tube as possible. The flexible tube, however, had to be about the same size as the neonatal endotracheal tube to be positionable, and thus could substantially block the endotracheal tube if it got too close. The resulting compromise in positioning the flexible tube near enough the endotracheal tube for an accurate sample but not blocking it, and the difficulty of achieving even this compromise, made the arrangement impractical.